Shear wave (S-Wave) seismic exploration techniques have historically employed shear wave seismic sources and shear wave seismic receivers in a seismic survey to gather seismic data. Such a seismic survey has been either linear or areal in its extent. The seismic energy imparted by the shear wave seismic source is detected by the shear wave seismic receivers after interacting with the earth's subterranean formations. Such seismic surveys, however, until recently have been limited to utilizing a shear wave seismic source having a single line of action or polarization, oriented with respect to the seismic survey line of profile, to preferentially generate seismic waves of known orientation, e.g., horizontal shear (SH) waves or vertical shear (SV) waves. The shear wave seismic receivers utilized in conjunction with a given shear wave seismic source have similarly been limited to a single line of action or polarization, oriented with respect to the seismic survey line of profile, to preferentially receive a single component of the seismic wave, e.g., (SH) wave or (SV) wave. As used herein, the term "line of action" generally comprehends a defined vector displacement, such as the particle motion of the seismic wave. In present shear wave seismic surveys, the lines of action of the seismic source and the seismic receivers usually have the same orientation relative to the line of profile and if so are said to be "matched".
The term "polarization" in the context of seismic waves refers to the shape and spatial orientation of particle trajectories. Here we restrict the term to mean only the spatial orientation of the line along which a particle moves in a linearly polarized wave. Hence "polarization" and "polarization direction", as used here, both imply the spatial orientation of such a line, the latter term emphasizing the restriction to linear rather than more general (e.g., elliptical) motion. A "polarization change", then, does not mean a change, for example, from linear to elliptical motion nor a polarity reversal but only a change in the spatial orientation of the line along which a particle moves.
As long as seismic surveys were limited to seismic sources and seismic receivers having a compressional (P) wave lines of action, satisfactory results were generally obtained irrespective of the orientation of the seismic survey line of profile with respect to the underlying geological character of the subterranean formations. However, when the seismic sources and seismic receivers are of the shear wave type, i.e., either horizontal shear (SH) wave or vertical shear (SV) wave, the orientation of the seismic survey line of profile and/or the line of action of the shear wave seismic source with respect to the geological character of the subterranean formations can determine whether or not meaningful seismic data is obtained.
As understood by those skilled in the art, compressional (P) waves are longitudinal waves where the particle motion is in the direction of propagation. Shear waves are transverse waves where the particle motion is in a transverse plane perpendicular to the direction of propagation. Two special classes of shear waves are defined herein. Specifically, horizontal shear (SH) waves where the particle motion in the transverse plane is further restricted to be perpendicular to the line of profile of the seismic survey (i.e., horizontal) and vertical shear (SV) waves where the particle motion in the transverse plane is further restricted to be perpendicular to the horizontal shear (SH) particle motion.
As the orientation of the seismic survey line of profile is dependent on the geological character of the subterranean formation, when matched shear wave seismic sources and shear wave seismic receivers are used, it is known by those skilled in the art that shear wave seismic surveys are adversely affected by azimuthally anisotropic subterranean formations. Azimuthally anisotropic subterranean formations are likely to have vertical planes of symmetry. Because shear wave behavior is complicated and generally uninterpretable when the symmetry planes are neither parallel to nor perpendicular to the line of action of the shear wave, care must be taken to ensure that the seismic survey line of profile is laid out either parallel or perpendicular to the symmetry planes.
When the seismic survey line of profile is laid out either parallel or perpendicular to the symmetry planes, the utilization of matched sets of (SH) wave and (SV) wave seismic receivers and seismic sources have provided useful information regarding the geological character of a subterranean formation. Such a technique requires prior knowledge of the seismic velocity anisotropy of the subterranean formation to be successful.
The interaction differences of (SH) waves and (SV) waves have been utilized to detect and measure the anisotropic properties of an azimuthally anisotropic subterranean formation when the seismic lines of profile are properly oriented with respect to the symmetry planes and matched sets of shear wave seismic sources and shear wave seismic receivers have been deployed in the seismic survey. In such applications, (SH) and (SV) shear wave seismic sources and seismic receivers are utilized, but only in matched sets, i.e., (SH) shear wave seismic sources with (SH) shear wave seismic receivers and (SV) shear wave seismic sources with (SV) shear wave seismic receivers. However, if the seismic survey line of profile is not properly oriented with respect to the planes of symmetry, the seismic information observed can be difficult to interpret at best.
The orientation of the seismic survey line of profile with respect to the symmetry planes is critical. Consequently, utilization of matched sets of shear wave seismic sources and shear wave seismic receivers have produced inconsistent results when the seismic survey line of profile has not been properly laid out with respect to the anisotropic geological character of the subterranean formations.
Those acquainted with the art of seismic exploration, especially in seismically virgin territory, realized that prior knowledge of the geological character of the subterranean formations is generally not available prior to seismic exploration. The method and system of geophysical exploration of the present invention can be advantageously employed without regard to or knowledge of the geological character of the subterranean formations and still obtain meaningful seismic data.
U.S. Pat. No. 3,302,164 relates to seismic exploration for detecting fluids in formations by obtaining a ratio of the velocities of shear waves and compressional waves along a seismic line of profile. In order for the ratio to be obtained, however, the frequency spectra of the waves introduced by a seismic source had to be controlled according to the average velocity ratio expected to be encountered. An article, "Combined Use of Reflected P and SH Waves in Geothermal Reservoir Exploration," Transactions of Geothermal Resources Council, Volume 1, May 1977, discussed tests made using both compressional and shear waves in exploring for and evaluating geothermal reservoirs. U.S Pat. No. 4,286,332 relates to a technique of propagating seismic shear waves into the earth from compressional wave producing vibrators. U.S. Pat. No. 4,242,742 describes a technique of obtaining shear wave seismic data from surveys where impact devices for waves are used as a seismic energy source.
S-wave birefringence, a property of elastic waves in anisotropic solids, is common for S-waves traveling vertically in crustal rocks. Early models of anisotropic sedimentary rocks proposed by exploration geophysicists were often transversely isotropic with vertical infinite-fold symmetry axes. Such solids are not birefringent for S-waves with vertical raypaths Earthquake seismologists (e.g., Ando et al., 1983; Booth et al., 1985), however, found near-vertical S-wave birefringence in earthquake data in the early 1980s. At the same time, oil companies recording three-component (3-C) seismic data independently found vertical birefringence in hydrocarbon-bearing sedimentary basins. (Winterstein). Researchers from Amoco, Exxon, Chevron and Colorado School of Mines documented this vertical birefringence for the first time publicly in 1986 at annual meetings of the EAEG and SEG (e.g., Alford, 1986; Willis et al., 1986; Becker and Perelberg, 1986; Frasier and Winterstein, 1986; Martin et al., 1986). Since then much additional evidence for vertical birefringence in sedimentary basins has accumulated (e.g., Squires et al., 1989).
A common model for vertical S-wave birefringence is extensive dilatancy anisotropy (EDA) proposed by Crampin et al (1984). The essential feature of this model is that horizontal stresses such as those from plate tectonics create vertically oriented, fluid filled cracks or microcracks which cause anisotropy that, unlike transverse isotropy with a vertical axis, will cause vertical S-wave birefringence. The validity of EDA as an explanation for vertical birefringence is not established, but it and variants of it have proved useful as a framework within which to record and interpret experimental data. An alternate model, which we call the Nur model (Nur, 1971; Nur and Simmons, 1969), proposes the unstressed rock is isotropic with a uniform distribution of randomly oriented cracks. Axial stresses preferentially close the cracks perpendicular to stress directions, making the rock anisotropic. It is almost certain, whatever the best model proves to be, that much of the observed vertical S-wave birefringence results in some way from horizontal stresses. Crampin and Bush (1986) also pointed out that vertical S-wave birefringence might provide a useful tool for reservoir development. The polarization direction of the fast S-wave in simple cases gives the direction of maximum horizontal compressive stress, a quantity much in demand by those who induce fractures in reservoirs by techniques such as hydraulic fracturing.
Available evidence, (discussed later), including offset VSP information supports the notion that the vertical S-wave birefringence is caused by horizontal stresses, and that the polarization direction of the fast S-wave lies in the direction of maximum horizontal compressive stress, even when subsurface structures are steeply dipping. It is likely however that rocks exist for which the polarization direction of the fast S-wave for vertical travel does not lie along the maximum horizontal stress direction. Rocks with fractures oriented by ancient stress regimes, or rocks of low symmetry with tilted symmetry axes, for example, might constrain the fast S-wave polarization to lie in a direction other than that of maximum horizontal stress.
Unmistakable evidence is hereby presented for major changes in S-wave polarization direction with depth (see also Lee, 1988). A relationship between these polarization changes and any change of horizontal stress direction certainly exists, and the S-wave birefringence data provide potentially useful information for reservoir development regardless what the relationship is. U.S. Pat. Nos. 4,803,666 and 4,817,061 (both to Alford) are hereby incorporated by reference. Alford discloses a method of determining the S-wave polarization angles by finding the angle at which S-wave energy on off-diagonal components of an S-wave data matrix was at a minimum. One implementation of Alford's method involves selecting time windows that include only the leading portions of the first arrival S-waves, and then calculating energy on the off-diagonal components at rotation angle increments of one degree.
However, an invalid assumption of Alford's rotation method is that S-wave polarizations along a given raypath are generally orthogonal. Such an assumption is strictly valid only in certain symmetry directions. The effectiveness of Alford's method is hindered by noise or by distortion of the signal on the off-diagonal components of the S-wave data matrix.
Accuracy of analysis by Alford's rotation method depends, at least in principle, on having signal amplitudes of off-diagonal XY and YX components identical at common times. If they are not identical, the data do not fit the model, and the matrix cannot be diagonalized by a single rotation of source and receiver coordinate frames. If signal on XY components differs systematically from that on YX components, there will be systematic errors in calculated azimuth angles. But changes of polarization with depth cause just such systematic differences in signal on XY and YX components; specifically, the signal on one of the two components lags that on the other by the amount imposed by the upper layer.
Lefeuvre et al. (1989) and Cox et al. (1989) used propagator matrices or transfer functions to analyze variations in S-wave birefringence with depth in multicomponent VSP data, instead of applicant's proposed method of layer stripping. These prior works utilize only a Fourier spectrum as an analytical method. Therefore, improvements in the S-wave data cannot be readily seen, and the quality of the improvements do not match applicant's results. Being able to see the improved wavelet (as with applicant's method) provides confidence to the analyst, as it provides information on how well the process is working.
Martin et al. (1986) analyzed changes in S-wave birefringence with depth in S-wave surface reflection data via a rudimentary layer stripping technique. They subtracted the effects of an upper layer to see the residual effects in a lower layer. Their approach, however, required the generally unwarranted assumption that symmetry planes in a deeper layer were orthogonal to those in an upper layer. That is, they did not perform any analysis to determine the actual orientation of the deeper symmetry planes.
Current methods of predicting subsurface fracture orientation or stress regimes fall short of providing accurate results, for the many reasons described above. There is therefore a need for an improved seismic method to evaluate changes in shear wave polarization with depth.